This invention relates generally to digital convergence of a cathode ray tube display, and in particularly to the selection of image elements for digital convergence.
In a digital convergence system convergence values are determined for each display color and applied at an array of points located at intersections of an alignment grid superimposed on a black background. In a manual digital convergence system a user may manually adjust deflection parameters at the grid intersections, in either or both horizontal and vertical scan directions, to superimpose individual colored lines to form a white grid on the black background. These adjustment values are then stored for each grid intersection and subsequently readout to provide a converged display image. During the convergence adjustment process the alignment grid display must be stable and free from transient geometric distortions, for example, resulting from instability of deflection fields forming the scanned raster, or raster size variation due to power supply loading effects causing the CRT ultor or high voltage supply to vary. In a CRT display, image brightness is directly related to the scanning beam current, where the darkest area represents the smallest current and the brightest the greatest beam current. Thus it may be appreciated that the beam current varies dynamically at video frequency rates. It is well known that a scanned raster size is inversely proportional to the square root of the of the high voltage or EHT applied to the second anode of the CRT. Hence if the high voltage supply exhibits poor voltage regulation, for example with beam current variation, the high voltage value will vary in response to the displayed image brightness, typically dropping with beam current increase. Thus, the scanned raster size will vary particularly in the area of any high brightness image content. Thus it may be further appreciated that brightness modulation of the raster size is undesirable during normal image display, and is particularly problematic if present during convergence alignment giving rise to convergence errors.
An apparatus for convergence correction in a TV receiver with a CRT display comprises a display screen with photo sensors positioned adjacent to screen edges. A cathode ray tube generates a raster for illuminating the display screen and the photo sensors. A signal generator generates a video signal having a first part for convergence alignment and a second part having sufficient video signal amplitude to cause an average level of beam current loading corresponding to an average level that can occur during broadcast video reception. The video signal being coupled to the cathode ray tube for illuminating the display screen and sensors. The video signal facilitating convergence alignment with the average level of beam current loading to ensure that convergence alignment is maintained during broadcast video reception.